I can't speak from first-hand experience, or for '08, but I know that Madden '07 was apparently really great. For a long time I was the highest rated game for the Wii with around an 8.7 if I remember correctly (granted, there were like 10 games out at the time, but still...).
I briefly looked up the ratings and '08 got slightly lower scores than '07. I would advise you to get '07 since it's apparently better. However, if you have to have the updated rosters, well, then EA already has you in it's grasp so just buy '08.